This invention relates generally to the measurement of various electrical parameters associated with the transmission of radio frequency (RF) power and more particularly to the measurement of the voltage standing wave ratio (VSWR) at a selected point along a radio frequency transmission line. A typical radio frequency transmission system comprises a transmitter, an antenna, and a transmission line for coupling the transmitter to the antenna. In order to realize maximum power transfer from the transmitter to the antenna, it is necessary to properly match the impedance of those two system components. Measurement of the existing impedance match is generally effected by inserting two directional couplers in the transmission line. One of the directional couplers develops a voltage E.sub.f indicative of the forward power and the other develops a voltage E.sub.r indicative of the reflected power. The ratio E.sub.r /E.sub.f defines a voltage reflection coefficient .rho.. The voltage standing wave ratio (VSWR), a generally employed indication of the quality of the impedance match, is defined by the expression VSWR = (1 + .rho.)/(1 - .rho.).
Certain known instruments for measuring forward and reflected power include the capability of measuring VSWR. However, the measurement of VSWR using these prior art instruments is cumbersome in that to do so variously requires side calculations, reference level setting, multiple meter movements, or conversion nomographs.
An improved standing wave ratio measurement instrument is described herein in which VSWR is continuously and automatically visually indicated to the user. In accordance with the illustrated preferred embodiment of the present invention two directional couplers are employed to produce voltages E.sub.f and E.sub.4 that are indicative of the forward power and reflected power, respectively. Voltages E.sub.f and E.sub.r are applied to analog circuitry for generation of a voltage or current that is directly proportional to a parameter known as return loss (RL.sub.dB). Return loss, expressed in decibel units, is defined by the expression RL.sub.dB = 20 log(1/.rho.) = 20 log(E.sub.f /E.sub.r). Return loss is related to VSWR by the expression RL.sub.dB = 20 log((VSWR + 1)/(VSWR - 1)). An analog meter scale is calibrated to indicate return loss in decibel units and/or VSWR directly and continuously, thereby eliminating the requirement, in certain prior art instruments, of first setting a reference power level and then actuating a function switch to obtain an indication of VSWR.
Other and incidental advantages of this invention will become apparent to those persons skilled in the art from an examination of the following detailed description and the accompanying drawings.